1. Technical Field
A distillation process and apparatus is disclosed for separating two or more components having different volatilities from a liquid mixture containing the components. The process and apparatus employs an improved heat pump configuration including multiple heat pumps and multiple reboiler heat exchangers in combination with a folded column design or modified dividing wall column design.
2. Description of the Related Art
Distillation is a method of separation that is based on a difference in composition between a liquid mixture and a vapor formed from the liquid mixture. This difference in composition arises from the dissimilar effective vapor pressures, or volatilities, of the various components of the liquid mixture. Distillation normally involves partial or flash vaporization of the liquid composition followed by condensation of the vaporized material, usually in multiple vaporization/condensation sections known as stages.
The apparatus in which the separating occurs is usually a vertical, cylindrical vessel called a distillation column. Fractional distillation or fractionation involves the collection of separate fractions of condensed vapor, each fraction being segregated thereby permitting an original liquid mixture to be separated into two or more product streams. Rectification involves the enrichment of a lighter stream by intimate contact with counter-flowing liquid reflux within the fractionation column, typically above the feed inlet, which removes heavier materials from the lighter stream. Stripping, on the other hand, involves the enrichment of a heavy stream below the feed by intimate contact with counter-flowing reboil vapors within the column, which removes lighter components from the heavier stream.
Distillation or fractionation columns normally contain internal devices for effecting vapor-liquid contact such as plates, trays or packings. Generally, the larger the number of plates or trays, the better the separation. A theoretical plate or a “theoretical stage” in a distillation process is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a mixture, establish equilibrium with each other. The performance of a distillation process depends on having a series of equilibrium stages and is therefore enhanced by increasing the number of theoretical stages (NTS).
The combining of two distillation columns into a single column having a vertical partition, known as a diving wall column (DWC), provides for the separation of the feed into a plurality of product streams. The use of DWCs can reduce the size or cost of the equipment needed to produce overhead, bottoms, and sidedraw products. DWC designs and control systems for DWCs are disclosed in U.S. Pat. Nos. 6,558,515 and 7,267,746. Despite the advantages of DWCs, their widespread use has not occurred.
A problem associated with distillation processes in general is the employment of relatively large pieces of equipment that are inefficient with respect to energy consumption. While conventional distillation systems employ a single reboiler at the bottom stage, it is known that energy efficiency could be increased by incorporating capital-intensive reboilers at multiple sections along the column. However, the capital cost of adding multiple reboilers to conventional distillation columns has been cost prohibitive.
Currently, with energy costs increasing, and with national and international pressures to reduce CO2 emissions, there is a recent compelling need for energy efficient distillation column configurations.